The `--file' Option

You can specify an argument for the `--file=archive-name' (`-f archive-name') option whenever you
use tar; this option determines the name of the archive file
that tar will work on.

If you don't specify this argument, then tar will examine
the environment variable TAPE. If it is set, its value will be
used as the archive name. Otherwise, tar will use the
default archive, determined at compile time. Usually it is
standard output or some physical tape drive attached to your machine
(you can verify what the default is by running tar
--show-defaults, see section Obtaining GNUtar default values). If there is no tape drive
attached, or the default is not meaningful, then tar will
print an error message. The error message might look roughly like one
of the following:

To avoid confusion, we recommend that you always specify an archive file
name by using `--file=archive-name' (`-f archive-name') when writing your tar commands.
For more information on using the `--file=archive-name' (`-f archive-name') option, see
Choosing and Naming Archive Files.